1. Field of the Invention
This invention relates to heat exchangers and more particularly to apparatus for cooling a fluid with abrasive and depositive characteristics.
2. Description of the Prior Art
In reactors for the gasification of carbonaceous materials such as coal, a combustible product gas is produced as well as solid waste products such as agglomerated ash. The untreated product gas from gasified coal is called raw gas and contains a significant amount of particles which are partially molten at the gasifier exit temperatures of approximately 1800.degree. F. These particles, which are of varying chemical composition, will stick both to metallic and non-metallic surfaces regardless of the angle of incidence of the gas flow to the surface when the gas flows out of the gasifier exit. It has been demonstrated that eventually flow passages will plug almost closed with solidified material.
Present information in technical papers and from experimental data indicate the deposition of these molten particles as they exit from the gasifier will not occur if one of the three following conditions are maintained: (a) the raw gas temperature does not exceed 1300.degree. F.; (b) the surfaces through which the raw gas passes or is allowed to impact are metallic and are maintained at less than 500.degree. F. at the gas/metal interface; or (c) the raw gas velocity is kept very low.
It has also been observed that very high erosion rates result from the abrasive nature of the raw gas. At times, particle quantities on the order of 800 lbs/hr. have been seen in the raw gas of a coal gasification unit which is rated at approximately 2500 lbs. of coal input per hour. These particles range in size from 2 microns to 300 microns and typical velocities range between 25 feet per second and 35 feet per second.
Since some erosion is inevitable, it may be necessary to replace those surfaces which are most severely eroded. Replacement of the entire heat exchanger is feasible but costly, so replacement of a smaller part of the heat exchanger would be less expensive both from the standpoint of component cost and replacement time.
It is also necessary to protect the tubesheet from exposure to the elevated temperatures of the raw gas.
It is thus desirable to provide raw gas heat exchangers with tubesheet structures which will be resistant to particle sticking and thus less susceptible to plugging, will be resistant to erosion and when undesirably eroded, may be replaced relatively easily and which will provide cooling for the heat exchanger tubesheet.